Crystalline silica (SiO2) generally represents silica of many different crystallization types such as α-quartz (low-temperature quartz), β-quartz (high-temperature quartz), α-cristobalite (low-temperature cristobalite), and β-cristobalite (high-temperature cristobalite). Crystalline silica often has stable chemical properties, high melting point, high mechanical strength, and resistivity to ultra violet light, and crystalline silica is an electrical insulation material.
Quartz particle and/or cristobalite particle are commonly used as a preferred raw material for applications in a variety of fields. For example, natural quartz particle and/or fused silica particles may be widely used in optical glass, electronic devices, packaging of very-large-scale-integrated (VLSI) circuits, electrical insulating, ceramics, paints, casting, medicine, cosmetics, rubber manufacturing, mobile communication, aerospace technology, etc. Meanwhile, high-purity quartz is the main raw material for forming monocrystalline silicon (single-crystal silica), polysilicon, quartz glass, optical fiber, solar cells, and substrates of integrated circuits. Industry often requires the degree of purity of the formed/prepared SiO2 to be considerably high. It is desirable that impurities such as certain metal ions (e.g., iron, titanium, chrome, zirconium, lithium, and sodium) and hydroxyl (—OH) may be significantly reduced or eliminated from the formed SiO2.
As electronic industry advances, requirements on the packaging materials of the VLSI circuit are becoming more demanding. For example, the silica particles are required to have desirably high degree of purity and degree of fineness, and a more concentrated distribution of particle size. The silica particles are required to be considerably fine, and with high-purity and low radioactive content.
Conventionally, quartz sand is often used as the material for preparing high-purity crystalline silica particles. By baking and granulating the raw material, and soaking the raw material in a certain acid, most of the foreign minerals and soluble impurities can be removed from the raw material and high-purity crystalline silica particles can be obtained. However, the silica particles prepared by the method described above often has a size and/or fineness greater than 2 μm, has an undesirably wide distribution of particles, and low sphericity, and is often highly radioactive. Metal contamination (e.g., iron and nickel) is often introduced into the silica particles. Also, high temperature (e.g., about 200 degrees Celsius to about 800 degrees Celsius) and/or high pressure (e.g., about 100 MPa to 3 GPa) conditions are also combined to prepare high-purity crystalline silica particles from a variety of raw materials (e.g., minerals and silica gel) by using different crystallization agents (e.g., NaOH). The prepared silica particles, however, may have one or more of the problems such as large particle size, severe particle aggregation, low sphericity, irregular shapes, long production cycles, high cost, highly radioactive, and low mono-dispersity.
In some conventional methods, when only water and silica are used to prepare high-purity crystalline silica without using any crystallization agent, high temperature and high pressure must be used, which of course requires high cost and long production cycles. Other conventional methods may include use of alkali metal hydroxide, which, however, may inevitably cause alkali metal pollution.
That is, the conventional methods for preparing high-purity crystalline silica particles may require high temperature, high pressure, and high requirements on the preparation equipment/apparatus, and the silica power prepared by the conventional methods still need to be improved.